Pilot study CCQM-P172 – spectroscopic methods for HNO₃ value assignment

Main text The CCQM-P172 comparison was designed to evaluate the level of compatibility of laboratories' spectroscopic methods for trace gas quantification using nitric acid (HNO3) as a model system. Because it is present in NO2 gas standards as an impurity, HNO3 was chosen as the analyte for study. As primary reference gas mixtures of HNO3/N2 are (generally) unavailable for calibration, traceability for spectroscopic measurements will rely on the use of referenced molecular parameters (such as tabulated in the HITRAN database, acronym for high-resolution transmission molecular absorption database). Laboratories that took part in this pilot study CCQM‐P172 were required to submit one secondary standard of NO2 in nitrogen at a nominal amount fraction of 10 μmol mol−1 with a requirement that the HNO3 amount fraction present in the mixture is between 100 nmol mol−1 and 1000 nmol mol−1. As the mixtures are similar to the ones compared in the Key Comparison CCQM-K74.2018, also coordinated by the BIPM during the same period, a similar linear decrease of the main component NO2 and a correlated linear increase of the major impurity HNO3 was initially foreseen. The CCQM-P172 protocol was designed to deal with standards that followed a well-behaved decay profile, allowing BIPM measurements to be compared to interpolated values for participants' standards. However, as with CCQM-K74.2018, the behavior of the standards deviated from this expectation, showing nonlinear variations of the components. The study was further complicated with deviations from the protocol which have been recorded and described in this report. The complications resulted in a reduced number of analyses by the coordinating laboratory for two of the five participants. The first version of this report distributed in May 2021 between participants described only the measurements performed by the BIPM and the results submitted by participants, without attempting to provide reference values. Nevertheless, a number of important observations and conclusions were drawn as reported below: 1) The BIPM operated two methods for FTIR quantification of HNO3, the first based on direct reference to HITRAN parameters, the second based on calibration with a permeation system. Whilst both methods demonstrate very good correlation and linearity with respect to each other, the HITRAN method provided measurement results 23% higher than for the permeation system calibrated FTIR method. 2) The fitting of HNO3 FTIR spectra was often complicated by the presence of other species, in particular the considerable NO2 absorption band for gas mixtures from cylinders, and the presence of considerable amounts of water in the case of the BIPM permeation calibration gas mixtures in the 1550 cm-1 to 1750 cm-1 spectral window. The effect of the spectral windows used for fitting deserves further consideration. 3) Stabilization of the FTIR signal in the BIPM system (45 m gas cell) with the flows of gas that are permissible from gas cylinders for HNO3 resulted in residual drift in signal that varied between standards submitted, and in the best cases was characterized by an additional uncertainty component with magnitude of 20 nmol mol−1 (standard uncertainty), and a compromise between increased signal sensitivity and its stability. 4) Due to the complications in the study, the set of results obtained, which had sufficiently complete sets of measurements to compare data, were for the cylinders received from NPL and VSL. In the case of the NPL standard, BIPM and NPL FTIR measurements were performed with similar HITRAN based methods, and agreement of results can be concluded if the rise in HNO3 levels can be considered to have stabilized in the standard at the time of measurement at the BIPM. The VSL method fits individual absorption lines within a different band from BIPM and NPL. For the VSL standards the absolute change in HNO3 amount fractions between their measurement periods is of the same order of magnitude as the uncertainties reported for the BIPM FTIR measurement methods. As a result both of the BIPM and the VSL results could be considered in agreement, without being able to readily differentiate for which of the BIPM results the agreement is better. During the GAWG meeting of June 2021, participants agreed to complete the study with the following actions: • reanalysis of participants' FTIR spectra when feasible: CENAM and NPL spectra were reanalysed using the submitted participants parameters, and the recalculated nitric acid mole fractions. Unfortunately, NMISA and KRISS could not provide enough information to reanalyse their spectra. VSL did not use FTIR for the analysis. The reanalysis confirmed good agreement with NPL and revealed differences with CENAM. • reanalysis using different regions: the impact of fitting HNO3 amount fractions in alternative regions, 1240 to 1400 cm-1 and 1240 to 1800 cm-1, was studied. The maximum difference in amount fractions calculated was dependant on the database used. • the Pacific Northwest National Laboratory (PNNL) database was used as alternative infrared database to retrieve HNO3 amount fractions of calibration mixtures produced by the permeation facility. A bias of 6.7 %, contrasting with 23 % identified using HITRAN, was found against HNO3 permeation-based values. Based on the measurements performed at the BIPM using FTIR calibrated with dynamic standards on one hand, and with molecular parameters found in the two databases HITRAN and PNNL, a strategy to further calibrate HNO3 values measured by FTIR in standards of NO2 in nitrogen is proposed. This is based on applying a correction to amount fraction values determined by the FTIR-HITRAN method, with the correction factor determined from the comparison with values from the Permeation system method, considered as the reference method. The corrected FTIR-HITRAN method, with traceability of values to the permeation method, can be applied reproducibly, provided that the FTIR gas cell's pathlength is regularly verified, and the spectral region fitted is defined. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database https://www.bipm.org/kcdb/. The final report has been peer-reviewed and approved for publication by the CCQM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).